In the particular case of an aircraft fuselage, a search is under way to reduce noise to improve passenger comfort and that of the air crew who are frequently for long periods subjected to an ambience at a high acoustic level caused by the engine, vibrations and air flows.
The frequencies are more particularly generated within ranges exceeding 500 Hz and more particularly between 1200 and 2000 Hz and these frequencies need to be treated as a priority as the organism of passengers is extremely sensitive to these.
The acoustic problems posed by aircraft fuselages also need to be resolved by suitably-adapted panels, but by taking account of complementary constraints, such as the weight, thickness, prices but also aesthetic constraints as these panels need to be able to be decorated on the internal face on the cabin side.
In a fuselage, there is one possible solution which consists of embodying an internal cock with heavy materials so as to limit transmission of noise, but this possibility is not readily compatible with the required lightness criteria.
Moreover, so as to obtain optimal efficiency, the cock needs to be sealed to avoid leaks, otherwise the acoustic treatment instead of reducing the intensity of the noise in the cabin tends amplify.
It is possible to measure the total noise volume, but this volume is not strictly harmful. On the other hand, the SIL (Speech Interference Level), which corresponds to a level of noise within a range of given frequencies for speech, is a highly important factor. It is necessary to reduce it and also important to reduce the level of noise in the SIL frequencies. In fact, the level of noise in certain frequencies may result in speech not being intelligible.
Another solution consists of disposing on the passage an absorbent material, such as an open-celled foam so as to reduce the acoustic level originating from the power unit. It is known that the noise propagates indirectly and is distributed in the cabin so it is also necessary to also treat this indirect noise and in particular that it is not amplified or reverberates onto the walls.
At the current moment in aircraft, these panels are provided with a high acoustic absorption coefficient and include at least one layer of foam so as to avoid the direct transmission of the source towards the inside of the cabin.
There are also decorative and aesthetic constraints as the face oriented towards the inside of the cabin needs to be covered so as to provide comfort and must possess a certain aesthetic aspect.
This interval face of the panel when seen and in physical contact with passengers need to be able to be cleaned easily when maintaining the inside of the cabin. Moreover, it is essential that the panel is sufficiently rigid so that a person can lean on it, even in cases of impact, without breaking it, this rigidity requiring that said panel possesses good mechanical resistance.
The solution retained by the invention consists of embodying covering panels with high acoustic attenuation, the aim of said panels being to dampen noise in the cabin and generated by the various links or acoustic bridges, it being known that an isolated leak-free cock cannot be easily embodied in a practical way for an aircraft cabin. It Is therefore necessary to have a panel able to resist reverberations, especially when the cabin is small, and a panel which is light and simple to produce so that the cost price remains within limits lower than those of existing panels.
Fire-resistance is another restraint to be observed for materials forming part of the walls of aircraft cabins. Therefore, it is essential that the components used when producing said wall observe the current standard FAR 25 .sctn. 853 (Fire/Fumes/Toxicity criteria) applicable to commercial aircraft.
In the various applications, documents exist of the prior art which concern rigid panels made of a composite material aimed at attenuating noises.
The patent FR-A-2.710.874 in the name of the Applicant concerns a rigid sheet self-supporting material for acoustic attenuation and a panel made of this material.
This material includes:
an a alveolar structural core, the alveolar being open, more particularly a honeycomb, PA1 an acoustically absorbent material filling the alveoles, PA1 strips of resistant fibres forming openings on each of the faces, and PA1 a wide mesh trellis covering the unit. PA1 a full sheet forming a casing, PA1 first spider type alveolar structural core, PA1 a first sheet made or a micro perforated metallic material with holes of between 50 and 500 .mu.m, PA1 a second spider type alveolar structural core, and PA1 a second sheet made of a micro perforated metallic material with upper permeability.
This type of panel is used as a transverse separation wall, especially in aircraft cabins. This panel is able to fully dampen the noise in the cabin, especially for turbopropeller engine type aircraft, but this panel is too heavy, too thick and especially less effective as a bulkhead covering, said bulkhead already generally including other acoustically absorbent materials.
The patent U.S. Pat. No. 3,439,774 describes a panel forming a housing for an aircraft reactor and intended to absorb the noise generated by the functioning of this reactor. To this effect, the following is superimposed from outside to inside
Thus, the high frequency noises propagate from inside to outside are preferably absorbed by the second sheet and the low frequencies are absorbed by the second sheet after treatment via passage through the second sheet.
This material is too thick to be used as a covering on a cabin wail. The material is thus revealed as being too heavy and this panel is limited to the particular application envisaged, namely the reduction of propagation of the noise originating from the aircraft engine.
Finally, the patent application DE-A-3.91 3.255 describes a panel including a plate including holes, a non-woven sheet, a honeycomb core, a plate fitted with holes with a smaller diameter and a thick mineral wool covering.
This panel includes an acoustic damper but it is too thick, too heavy and too complex to be applied to aircraft cabin internal covering panels.
The present invention also concerns a covering and/or self-supporting panel with acoustic damping, which is light, mechanically resistant and sufficiently simple to produce so that its cost price is compatible with the application to the aircraft cabins and whose fire-resistance satisfies current standards and whose internal face on the cabin side can be decorated.
The present invention also includes a method for producing said sandwich panel and more specifically method for producing one of the skins of the sandwich.